1. Field of the Invention
This invention relates to a semiconductor device, particularly to a bipolar type transistor having a meshed plane pattern emitter region.
2. Description of the Prior Art
Bipolar type high power, high frequency transistors have been put into practical use in the form of overlay type transistors, mesh emitter transistors and ring emitter transistors.
Overlay type transistors are constructed of a plurality of emitter regions formed as islands in the vicinity of the base region surface and are connected in parallel by the electrodes.
Mesh emitter transistors have meshed emitter regions formed in the vicinity of the base region surface and electrodes are led from the intersections of the meshed portions.
Ring emitter transistors have a plurality of ring-shaped emitters deposited in the form of islands in the vicinity of the base region surface and the ring-shaped regions are connected in parallel by the electrodes.
In these high power, high frequency transistors, the periphery of the emitter region is made as large as possible in the limited base area in order to realize higher power and high frequency characteristics.
This invention relates to an improvement in high power and high frequency transistors having a mesh emitter structure. Such mesh emitter transistors have a structure as shown in FIG. 1. FIG. 1(b) is a sectional view of FIG. 1(a) taken along the line A--A'.
In FIG. 1, 1 is an N type silicon substrate forming the collector region; 2 is a P type base region which is formed by diffusing the acceptor impurity into the main surface of the silicon substrate 1 either selectively or over the entire surface; 3 is an N+ type emitter region formed by selectively diffusing the donor impurity into the P type base region 2. The N+ type emitter region 3 has the mesh type planar surface. Further, 4 is an insulating film, consisting of silicon dioxide (SiO.sub.2), etc., covering the surface of the silicon substrate; 5 is the base leadout electrode; 6 is the emitter leadout electrode; and 7 is the collector electrode. In addition, 12a, 12b, . . . , are base electrode windows provided in the insulating film 4 and 13a, 13b, . . . , are emitter electrode windows provided in the insulating film 4. 14a, 14b, . . . , are P+ type base contact regions (base compensating diffused regions).
A mesh emitter transistor having such a structure is often used as a switching regulator, DC-DC convertor because of its high speed switching characteristics. For applications such as a switching regulator and DC-DC convertor, however, the mesh emitter transistor is required to have a high breakdown voltage and a large safe operating area because it operates on a large current.
Consequently, the emitter strips of the mesh emitter transistor are widened to reduce the current density per unit area of the emitter region. When the emitter strips are widened, however, the sectional area of the base region is reduced due to the enlargement of the emitter region and a resistance of the base region under the emitter region 3, namely an internal base resistance rbb' in the direction from almost the center of the intersections of the emitter strip to the base contact region 14 (the direction along the line A--A' of FIG. 1(a)) increases.
Such an increase in the internal base resistance results in a deterioration of the switching speed of the mesh emitter transistor and causes part of the charge to remain just under the emitter strip during the switching operation. This is particularly so during turn-off, thereby causing a concentration of current and a resultant breakdown of the mesh emitter transistor.